1. No category

1938-13
Workshop on Nanoscience for Solar Energy Conversion
27 - 29 October 2008
Reduced and Doped TiO2: what is the nature of the defect states?
Gianfranco PACCHIONI
Universita degli Studi di Milano-Bicocca
Dipt. Scienza dei Materiali
Via R. Cozzi 53
20125 Milano
Italy
Workshop on “Nanoscience for solar energy conversion”, Trieste, October 27-29, 2008
Reduced and doped TiO2: what is the nature of the
defect states?
G. Pacchioni, E. Finazzi, C. Di Valentin
Dipartimento di Scienza dei Materiali
Università Milano-Bicocca, Milan (Italy)
A. Selloni
Department of Chemistry,
Princeton University, Princeton (USA)
A. Czoska, M. C. Paganini, M. Chiesa,
S. Livraghi, E. Giamello
Dipartimento di Chimica IFM,
Università di Torino (Italy)
TiO2 polymorphs
Anatase, Eg = 3.2 eV
Ti-Ti
3.79 Å
3.04 Å
rutile, Eg = 3.0 eV
Ti-Ti
3.57 Å
2.96 Å
TiO2: very important role of defects
Color change in TiO2 samples induced by
incresing level of defects
Defects change electronic properties of
the material
Defect engineering of TiO2 for
photocatalytic applications
Main defects: oxygen vacancies, Ti3+ ions
OUTLINE
Part I : intrinsic defects
O vacancies and Ti interstitials
Part II : extrinsic defects
N impurities and their interplay with other defects
What is the nature of “reduced titania”?
Excess of metal compared to oxygen results in formation of
“Ti3+” ions. But …
TiO2-x
Ti1+xO2
O vacancies
Ti interstitials
Experimental
proofs of
formation of
reduced bulk
titania:
(1) UV-vis
(2) UPS
(3) XPS
(4) EPR
Every theoretical
method dealing with
reduced TiO2 should be
able to reproduce these
features…
(1) absorption bands in the
visible region responsible for
the change in color (assigned
to d-d transitions)
(2) occurrence of a new
state in the gap at about
0.8 eV below the
conduction band attributed
to the reduced Ti3+ ions
(3) shift in the core level
binding energies of the
reduced Ti atoms from Xray photoemission (XPS)
(4) presence of more
than one EPR signal
associated to various
kinds of paramagnetic
Ti3+ ions in the lattice
Ti4+
Ti3+
DFT calculations
QUANTUM EXPRESSO (pure DFT, DFT+U)
plane-wave-pseudopotential approach
PBE functional – spin polarized calculations
96-atoms supercells
anatase
CRYSTAL06 (hybrid DFT)
atomic gaussian basis set approach
Hybrid DFT functionals – spin polarized calculation
hyperfine coupling constants (EPR)
rutile
O vacancy in bulk anatase: standard DFT
Eg 2.6
eV
No states in the gap, electrons fully delocalized, no major
geometrical distortion around the defect
Problem due to self-interaction in DFT
Pragmatic ways to improve the model
DFT+U methods
Systems where atomic-like features are persistent in the solid:
explicit description of correlation by an on-site correlation term U
for a few selected localized orbitals
Add a Hubbard-like EHub term to the standard functional:
Anisimov et al., Phys. Rev. B 44, 943 (1991)
Hybrid functionals
Use of hybrid functionals where HF-like exchange is mixed in with the DFT
exchange: B3LYP (20%), H&H LYP (50%)
Becke J. Chem. Phys. 98, 5648 (1993)
O vacancy in bulk anatase: B3LYP and DFT+U
Remove an O atom: 2e left on the system.
Quantities of interest:
(1) Energy gap (Eg)
(2) Distance of impurity levels from bottom of
conduction band ('E1, 'E2)
B3LYP
'E1 1.2 eV
'E2 1.3 eV
Both electrons
are localized on
Ti 3d levels ...
DFT+U
Ti3+
Eg
3.9
eV
(U = 3 eV)
Eg
2.8
eV
'E1 0.6 eV
'E2 0.9 eV
...but
delocalized
solution is close
in energy
Finazzi, Di Valentin, Selloni, GP, J. Chem. Phys. 128, 182505 (2008)
Similar results for rutile (110) surface
B3LYP xc functional, c(4x2) surface cell, AO Gaussian basis sets, spin
polarized calculations, CRYSTAL03
O vacancy: two
electrons localized
on two nonequivalent Ti ions
triplet ground
state (magnetic)
Two states in
the gap at 1.2 and
0.9 eV below
conduction band
(as in the
experiment)
Di Valentin, GP, Selloni, Phys. Rev. Lett. 97, 166803 (2006)
Polaronic nature of O vacancy
O vacancy in rutile
TiO2(110): strong
distortion around the
defect (similar for
hydroxilated surface,
where Ti3+ forms)
Geometry: B3LYP
DOS: B3LYP
localization
Geometry: B3LYP
DOS: PBE
localization
Geometry: PBE
DOS: B3LYP
no localization
Distortion essential to
induce localization but
present only using
hybrid DFT methods
Geometry: PBE
DOS: PBE
no localization
Di Valentin et al. Chem. Phys. 339, 44 (2007)
Reduced titania: a problem for theory
Pure functionals fail to reproduce the correct band structure
DFT+U or hybrid functionals improve the description but suffer from
the use of “empirical” parameters
Need to compare different approaches
Need for clean experimental data (often missing)
Generate defects in controlled way and characterize them by various
spectroscopies (central role of EPR)
In summary ...
F and Nb doping of stoichiometric TiO2 leads to lattice
Ti3+ ions in pseudo-octahedral enviornment
H doping of stoichiometric TiO2 leads to the same lattice
Ti3+ ions
Reduction of stoichiometric TiO2 in mild conditions leads
mainly to lattice Ti3+ ions
Strong evidence from comparison of measured and
computed EPR spectra of localized Ti 3d states at low T
Only hybrid or DFT+U functionals lead to electron
localization
Localized and delocalized states are very close in energy
and can depend on external conditions (T, P, etc.)
New experiments: delocalization is a temperature effect?
Delocalization of trapped electron occurs at RT (but here good
agreement for the wrong reason: DFT calculations done at 0 K!)
Barrier for e migration from site to site <0.1 eV (see M. Dupuis
lecutre). At RT e is mobile on the time scale of experiment
Reduced titania: oxygen vacancies versus Ti interstitials
increasing annealing temperature o
surface
TiO2
TiO2x
TiO
Ti
subsurface
TiO2
TiO2x
TiO2x
TiO
bulk
TiO2
TiO2
TiO2x
Ti1+xO2
Stoichiometric
sample,
colorless
O diffusion from bulk to
surface
O vacancies form in subsurface
regions
Lattice Ti3+ (localized states at
low T)
Ti diffusion
from surface
to bulk
Ti interstitials
form in
subsurface
regions
Titania in photoelectrolysis and photocatalysis
APPLICATIONS
(3) Final products:
CO2 and Water
(1) Organic pollutant
adsorbes to surface
(2) Adsorbed pollutant
breaks down
photocatalyst
- Photoelectrolysis of H2O
with TiO2 electrode (1970s)
- Photocatalytic production
of H2 (1980s)
- Photocatalytic oxidation of
organic molecules (1990s)
- Photoinduced hydrophilicity
- Self-cleaning, air and water
purification, self-sterilizing
- Band-gap photoexcitation
- Electron-hole
recombination
- Migration to surface
- Charge carrier trapping
- Charge transfer
J.T. Yates Jr. et al. , Chem. Rev. 1995
Photocatalysis by pure TiO2
B-x
e-
E
cb
B
B-x
e-
E
cb
B
hQ (UV)
A
A+x
Photocatalysis by doped TiO2
h+
Pure TiO2: band gap does
not allow to capture most
part of solar light (only a
small portion of the solar
spectrum is absorbed)
vb
A
A+x
hQ (VIS)
h+
vb
Reduction of band gap
obtained by inclusion of
dopants: transition metals
(V, Cr, Mn, Fe) or main
group atoms (N, C, S, F)
Properties of sol-gel prepared anatase N-doped TiO2
EPR
TiO2
Sim.
Sper.
N-TiO2
3300
3320
3340
3360
3380
3400
B/Gauss
TiO2
%R
N-doped Anatase
Anatase
U.V.
N-TiO2
CB
Two paramagnetic
species detected (EPR)
- Weakly physisorbed
molecular NO
- Stable bulk species
(responsible for yellow
color)
a.u.
Abs
Vis.
VB
400
300
400
500
nm
600
700
800
UV-vis absorption spectra of pure
and N-doped anatase
500
0
600
nm
1
700
800
2
3
Time (h)
4
5
Rate of methylene-blue degradation on
N-doped (a) and pure (b) anatase
DOPED TiO2: WHERE ARE THE DOPANTS?
interstitial (bound to lattice O)
or
substitutional (to O or Ti atoms)
Distinction possible by XPS? Simple (but often misleading) interpretation:
Peak at 400-401 eV:
interstitial N (N-O, N-H,
etc.)
Peak at 396-397 eV:
substitutional N (Ti-N)
XPS not always conclusive
NITROGEN DOPED ANATASE: DFT (PBE)
substitutional
Nitrogen Nsub
O
N
O
Ti
O
O
Ti
interstitial
Nitrogen Nint
O
Ti
O
O
Ti
O
O
O
Ti
Ti
O
O
N
Ti
N 2p
0.14 eV
vb
O
Ti
Ti
O
O
O
Ti
Ti
O
2.6 eV
2.6 eV
Ti
O N
cb
cb
Ti
O
O
Ti
Ti
S* N-O
N
Ti
0.73 eV
vb
Both interstitial and substitutional N (paramagnetic) introduce localized states
above the O 2p valence band
Substitutional versus interstital N-doping: EPR
Ti
Ti
N
Ti
Nsub
EPR
A1(N), G A2(N), G
A3(N), G
exp
2.4
4.2
32.5
Nint
0.2
1.8
33.4
Nsub
2.5
2.8
38.2
O
Ti
Ti
N
Ti
Nint
EPR: confirms localized nature of N-induced states in the gap
Theory: PBE (Espresso) and B3LYP (Crystal03) calculations show similar
level of localization. EPR properties computed at B3LYP level
Hyperfine coupling constants: interstitial Nint more likely than
substitutional Nsub (consistent with XPS)
Di Valentin, Pacchioni, Selloni, Livraghi, Giamello, J. Phys. Chem. B 109 (2005) 11414
Messagge from EPR and theory:
Stable N-species exist in the bulk
Most likely interestitial N is formed
N-induced states are localized!
A conclusion of purely theoretical
interest?
Interplay between N-doping and oxygen vacancies
DFT predicts: N-doping favors
vacancy formation
Energy cost to form Vo
pure TiO2: 4.2 eV
N-doped TiO2: 0.6 eV
Ns-doping stabilized by the
presence of oxygen vacancies
(NS-O+VO)
most stable situation under
oxygen-poor conditions
Di Valentin, Pacchioni, Selloni, Livraghi, Giamello, J. Phys. Chem. B 109, 11414 (2005)
Di Valentin, Pacchioni, Selloni, Chem. Materials 17, 6656 (2005)
Interplay between Ti3+ states and N-impurities
cb
Nbx
3d1 Ti3+
3d0 Ti4+
Nb
vb
DFT predicts: spontaneous charge transfer from Ti3+(3d)1 to Nb
formation of Ti4+ and Nb centers (singlet state, lower by 3 eV than triplet state)
Nb centers introduce states above O 2p valence band as neutral Nb states
Di Valentin, Pacchioni, Selloni, Livraghi, Giamello, J. Phys. Chem. B 109 (2005) 11414
Experimental proof: reaction of TiO2 with atomic N
Nx
Reduced TiO2 powders exposed to
atomic N: decrease of Ti3+(3d)1
signal until it is completely
quenched
Chiesa, Giamello (2008)
g3 = 2.003
Ti3+
Batzill, Morales, Diebold, Phys.
Rev. Lett. 96, 026103 (2006)
Increasing N exposure
TiO2 single crystal: N
implantation reduces peak
due to Ti3+(3d)1 states in UPS
3000
A3 = 3.23 mT
333
334
335
336
337
338
339
340
B / mT
341
342
a
b
c
d
Nx
3100
3200
3300
3400
e
3500
B / Gauss
3600
3700
3800
3900
An interesting experiment on N-doped samples
N b
Blue
+ O2
Lamp Lamp
off
off
Blue
Blue
3.5
I/I 0 3.0
(a.u.)
2.5
EPR of N-doped
TiO2 under irradiation
Reversible creation
of Nbx paramagnetic
centers (b = bulk)
Lamp
off
2.0
1.5
Green
O2x
1.0
0.5
0.0
0
50
100
Time (min)
150
Light induces
conversion of
diamagnetic
precursors into
paramagnetic centers
Irradiation under O2: O2 captures one electron from TiO2 surface and forms O2O2 acts as electron scavanger
Livraghi, Paganini, Giamello, Selloni, Di Valentin, Pacchioni J. Am. Chem. Soc. 128, 15666 (2006)
Generation of paramagnetic defects under irradiation
N-doped TiO2
e
CB
vis-hQ
Nb
Nb
VB
Process completely reversible by turning light off
Adsorbed O2 as electron scavanger
N-doped TiO2
O2
e
CB
vis-hQ
Nb
O2 x Nbx
VB
Process irreversible under O2 exposure; formation of O2
N-states responsible for photo-activity of N-doped TiO2
EPR and DFT show unambiguously the localized nature of N-induces
states in TiO2
N dopants favor formation of oxygen vacancies in the material (Ti3+
states); vacacies can become hydroxilated (same behaviour)
Charge transfer occurs from Ti3+ states (high in the gap) and singly
occupied N-states (low in the gap): Ti3+ + Nbx o Ti4+ + Nb
Visible light induces conversion of diamagnetic precursors into
paramagnetic centers by excitation of electrons to the conduction band
(Nb + hQ o Nbx)
Excited electrons are captured by O2 adsorbed on the surface
(formation of paramagnetic O2)
N-states responsible for photo-activity of N-doped TiO2
Livia
Giordano
Lorenzo
Ferraro
Cristiana
Di Valentin
Annabella
Selloni
Matteo
Frigerio
Fabrizio
Cinquini
Sabrina
Sicolo
Umberto
Martinez
Elio
Giamello
Emanuele
Finazzi